In some RF transmission systems, it is desirable to use a VGA. For example, the gain may be varied for varying output power or used for gradual power ramping to limit signal energy spill-over in neighboring channels. FIG. 1 shows one type of conventional variable gain amplifier 10.
U.S. Pat. No. 7,889,006 to Jones (“Jones”) discloses a VGA as drawn in FIG. 1. In FIG. 1, differential input signals Vin+ and Vin− are applied to the transistors Q1 and Q2. Fixed current sources I_DC1 and I_DC2 couple the emitters of the transistors Q1 and Q2 to ground. A resistor R1 is coupled across the emitters to set a minimum gain of the amplifier. In parallel with resistor R1 are resistor R2, a MOSFET M1, and a resistor R3. When MOSFET M1 is off, the minimum gain is achieved. Progressively turning on MOSFET M1 increases the gain to a maximum. A VCTRL signal is converted to a proper level by a gate voltage control circuit 12.
The currents through the transistors Q1 and Q2 cause voltage drops across the resistors R4 and R5, and the output of the amplifier may be the differential voltages Vout− and Vout+.
One drawback of amplifier 10 is that it has a limited headroom. In this context, the term headroom is the difference between the minimum input voltage Vin level needed for proper operation and Vcc (the operating voltage). Specifically, a problem with the circuit of FIG. 1 is that Vin cannot go below the level needed for the current sources to supply the target current.
FIG. 2 illustrates a very simple example of the current source I_DC1 or I_DC2 in FIG. 1. A fixed bias voltage Vbias is coupled to the base of the bipolar transistor 14, and a degeneration resistor 16 is provided for improving the noise performance of the current source. In a typical amplifier 10, the voltage drop across the resistor 16 may be 100 mV. For linear operation of the transistor 14, the collector-emitter voltage Vce may be a minimum of 0.4 volt. Therefore, the collector voltage must be at least 0.5 volt. Referring back to FIG. 1, the minimum level of Vin+ or Vin− must then be at least 1.2 volts, assuming a Vbe of the transistors Q1 and Q2 of 0.7 volt. Vcc must be high enough so that the Vce of the transistors Q1 and Q2 is sufficiently high for linear operation even with the highest currents across the transistors Q1 and Q2. As seen, due to the minimum operating requirements of the current sources I_DC1 and I_DC2, the minimum Vcc necessary to properly operate the amplifier 10 is limited.
Further, since the Vin+ and Vin− signals are generated by an upstream circuit, such as a digital-to-analog converter (DAC), the relatively high required minimum Vin+ and Vin− signals lower the headroom for those other circuits.
It is desirable to operate battery powered transmitters at lower and lower voltages. Therefore, what is needed is a highly linear VGA that enables an input voltage to be lower than the minimum input voltage of the circuit of FIG. 1. This will increase the headroom of the amplifier and the headroom of circuits that supply input signals to the amplifier. Thus, the transmitter system can be operated with a lower operating voltage Vcc.